A "galactic lens" has revealed that the Universe will probably expand forever.

Astronomers used the way that light from distant stars was distorted by a huge galactic cluster known as Abell 1689 to work out the amount of dark energy in the cosmos.

Dark energy is a mysterious force that speeds up the expansion of the Universe.

Understanding the distribution of this force revealed that the likely fate of the Universe was to keep on expanding.

It will eventually become a cold, dead wasteland, researchers say.

The study, conducted by an international team led by Professor Eric Jullo of Nasa's Jet Propulsion Laboratory in California, is published in the journal Science.

Dark energy makes up three-quarters of our Universe but is totally invisible. We only know it exists because of its effect on the expansion of the Universe.

To work out how dark energy is spread through space, astronomers used the Hubble Space Telescope to observe the way that light from distant stars was distorted around Abell 1689, a nearby cluster of galaxies.

Abell 1689, found in the constellation of Virgo, is one of the biggest galactic clusters known to science.

Light bends around massive galaxy clusters, allowing distant objects to be seen

Because of its huge mass, the cluster acts as a cosmic magnifying glass, causing light to bend around it.

The way in which light is distorted by this cosmic lens depends on three factors: how far away the distant object is; the mass of Abell 1689; and the distribution of dark energy.

The astronomers were able to measure the first two variables using the European Southern Observatory's Very Large Telescope, enabling them to calculate this crucial third factor.Cold comfort

Knowing the distribution of dark energy tells astronomers that the Universe will continue to get bigger indefinitely.

Eventually it will become a cold, dead wasteland with a temperature approaching what scientists term "absolute zero".

Professor Priyamvada Natarajan of Yale University, a leading cosmologist and co-author of this study, said that the findings finally proved "exactly what the fate of the Universe will be".